The present invention relates to the chemical vapor deposition (CVD) production of diamond by combustion flame techniques and more particularly to improving the carbon utilization rates of such techniques.
Its hardness and thermal properties are but two of the characteristics that make diamond useful in a variety of industrial components. Initially, natural diamond was used in a variety of abrasive applications. With the ability to synthesize diamond by high pressure/high temperature (HP/HT) techniques utilizing a catalyst/sintering aid under conditions where diamond is the thermally stable carbon phase, a variety of additional products found favor in the marketplace. Polycrystalline diamond compacts, often supported on a tungsten carbide supports in cylindrical or annular form, extended the product line for diamond additionally. However, the requirement of high pressure and high temperature has been a limitation in product configuration, for example.
Recently, industrial effort directed toward the growth of diamond at low pressures, where it is metastable, has increased dramatically. Although the ability to produce diamond by low-pressure synthesis techniques has been known for decades, drawbacks including extremely low growth rates prevented wide commercial acceptance. Recent developments have led to higher growth rates, thus spurring recent industrial interest in the field. Additionally, the discovery of an entirely new class of solids, known as "diamond like" carbons and hydrocarbons (DLC films), is an outgrowth of such recent work. Details on CVD processes additionally can be reviewed by reference to Angus, et at., "Low-Pressure, Metastable Growth of Diamond and `Diamondlike` Phases", Science, vol. 241, pages 913-921 (Aug. 19, 1988); and Bachmann, et al., "Diamond Thin Films", Chemical and Engineering News, pages 24-39 (May 15, 1989).
Low pressure growth of diamond has been dubbed "chemical vapor deposition" or "CVD" in the field. Two predominant CVD techniques have found favor in the literature. One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1,750.degree. to 2,400.degree. C. The gas mixture disassociates at the filament surface and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is held in a resistance heated boat (often molybdenum) and heated to a temperature in the region of about 500.degree. to 1,100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density, growth rate, and it is believed to enhance formation of diamond films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
A third technique not favored in the literature involves the generation of a combustion flame from the hydrocarbon/hydrogen gaseous mixture in the presence of oxygen. A drawback to this technique is the very low (0.01%) carbon utilization rate. However, because this rate is so low, a small absolute improvement (e.g. even 1%) in carbon utilization rate could reduce the operation cost and capital cost of the combustion flame process by two orders of magnitude so that such a modified combustion flame process could economically compete with, for example, the hot filament process which has a carbon utilization rate of about 30%.